1. Field of the Invention
The present invention relates to a semiconductor device and, in particular, to a structure of a ceramics package with a semiconductor element sealed therein.
2. Description of the Related Art
Conventionally, ceramics, resin, insulators, etc., have been employed as a principle material for a package with a semiconductor element, such as an IC and an LSI element, incorporated therein. Further, a heat sink is often attached to the package to radiate heat evolved from the semiconductor element.
FIG. 1 is a cross-sectional view showing a conventional semiconductor device. The semiconductor device is housed in a ceramics package. FIG. 2 is a bottom view of a ceramics package as seen from the bottom side of the semiconductor device shown in FIG. 1, that is, a view of the ceramics package with a cap, a semiconductor element and connection leads omitted. The cross-sectional view of FIG. 1 corresponds to that as taken along line 1--1 in FIG. 2.
As shown in FIGS. 1 and 2, a ceramics plate 2 made of alumina (Al.sub.2 O.sub.3 90 wt %) constitutes a package. A multi-layered interconnection pattern, not shown, is formed at those areas of the lower surface and interior of the ceramics plate. The ceramics plate 2 is provided by stacking a plurality of green sheets with a conductive layer coated to provide the interconnection pattern on their surfaces and baking a resultant structure. An opening 21 is provided at the central area of the ceramics plate. A heat sink 3 made of a Cu-W alloy (Cu 10 wt %) is so fitted in the opening 21 from above the ceramics plate 2 as to cover the opening 21 of the ceramics plate 2 with the edge of the heat sink 3 attached to the upper edge of the ceramics plate 2 at an area of the opening 21. A bonding agent layer 4, such as a silver solder, is interposed at a contact area between the heat sink 3 and the ceramics plate 2 to join the heat sink 3 to the ceramics plate 2. An attaching screw 32 is provided on the upper surface side of the heat sink 3 whereby a heat radiating fin unit 8 made of aluminum is attached to the heat sink 3.
A semiconductor element 1 is joined to the lower surface of the heat sink 3 by a bonding agent not shown. Connection electrodes 11 are provided on the semi-conductor element 1 and electrically connected to the interconnection pattern by the connection leads 6. As the connection method use is made of a wire bonding method using Al wires and Au wires, a TAB (Tape Automated Bonding) method using a tape carrier, and so on.
The semiconductor element 1 is sealed with a cap 7 made of a Fe-Ni-Co alloy, etc., and a sealing ring 22. The sealing ring 22 is fixed to the lower surface of the ceramics plate 2 by a bonding agent, not shown, such as a silver solder and jointed to the cap 7 by a bonding agent, not shown, such as an AuSn eutectic solder. Connection pins 5 serving as external terminals of the semiconductor device are mounted erect at the lower surface portion of the outer edge portion of the ceramics plate 2 and electrically connected to the interconnection pattern.
The semiconductor device is mounted to a circuit board, not shown, such as a printed circuit board, where connection electrodes are provided. The forward ends of the connection pins 5 on the semiconductor device are contacted, under heating and pressure, with the connection electrodes. In this way, the semiconductor device is mounted on the surface of the circuit board. Although the connection pins 5 are arranged in five arrays in the conventional device, there has been a tendency that more arrays are adopted with an increase in the high integration density of the semiconductor element.
With the semiconductor device operated, heat is evolved in the semiconductor element. For such heat evolved, the heat sink 3 and heat radiating fin unit 8 are provided for adequate heat radiation. The evolution of the heat expands the ceramics plate 2, heat sink 3, etc. The extent of the thermal expansion differs from member to member. In the semiconductor device with the respective members bonded and assembled in various combinations, mechanical or thermal stress tends to be concentrated to a specific area, sometimes causing such a stressed member to a breakage. In order to prevent concentration of the stress, use has to be made of those materials whose thermal expansion coefficients are near to each other.
For the conventional semiconductor device it is considered that, as a material near in thermal expansion coefficient (28.times.10.sup.-7 /.degree.C., 0.degree. to 200.degree. C.) to silicon of the semiconductor element 1 proper, for example, alumina with a thermal expansion coefficient of 66.times.10.sup.-7 /.degree.C. (0.degree. to 200.degree. C.) is used for the ceramics plate 2 and a Cu-W alloy (Cu 10 wt %) with a thermal expansion coefficient of 65.times.10.sup.-7 /.degree.C. (0.degree. to 200.degree. C.) is used for the heat sink 3. The selection of such materials can prevent generation of the stress to some extent. With a recent rapid tendency to a high speed/high integration density of a semiconductor device, such a countermeasure is not adequate in a present situation where the size of the semiconductor element is as large as over 17.5 mm square and a dissipation power reaches 20 W for a class using over 800 connection pins.
Referring to FIGS. 3A and 3B, explanation will be given below about the manner in which a crack occurs in a ceramics plate of a semiconductor device due to such stress involved. FIG. 3A is a plan view showing a ceramics plate and a heat sink mounted to the ceramics plate and FIG. 3B is a cross-sectional view as taken along line 3B--3B in FIG. 3A.
The ceramics plate 2 has an opening 21 at its central area and a heat sink 3 is fixed by a bonding agent layer 4 to the inner surface and lower surface edge portion of the opening 21. If the semiconductor device, not shown, is operated in that state, the ceramics plate 2 and heat sink 3 become elongated due to heat involved. Since, in this case, both are so selected as to be near in their thermal expansion coefficients to each other, their elongations differ due to some difference caused between the ceramics plate and the heat sink 3. This produces a warp in the ceramics plate 2 and stress is concentrated in an easily affected spot, thus producing a crack 9 there. Naturally, stress is also developed when the semiconductor device is cooled.
With the semiconductor device as shown in FIG. 1, the semiconductor element 1 is mounted to the surface of the heat sink 3 by an electroconductive bonding agent such as a Pb-Ag solder and silver paste. Upon mounting, however, any excess electroconductive bonding agent reaches the interconnection pattern on the surface of the ceramics plate 2, thus posing an interconnection short-circuit problem.